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Message: <blockquote data-quote="Ray Clark" data-source="post: 518953" data-attributes="member: 31103">As I found out with my inexpensive compression tester, quality of tester can make a big difference in accuracy.When I tested my old Johnson 30 HP using my lowest-cost compression tester, I only measured 70 PSI in each cylinder; both cylinders measured exactly the same.  However, it would start and would run, but I was having issues keeping it going at idle and low speed.  The problems I was having with that motor were not likely compression related.I suggest that if all cylinders measure within 10% of each other, and it's running good at high speed, then your problem is not likely to be compression related.</blockquote>